1. Technical Field
This disclosure relates to sheet metal forming tools and processes that are used to form and trim a hem flange or weld flange.
2. Background Art
Vehicle body panels such as deck lids, hoods, doors and the like frequently include a flange that extends about their periphery. Such body panels have traditionally been manufactured from mild steel sheet metal. Mild steel is very ductile and is easily formed in a hem forming operation. Increasingly, automotive manufacturers are turning to aluminum or advanced high strength steel (AHSS) alloys to obtain weight savings for vehicle body panels. Aluminum alloys and AHSS alloys offer high strength/low weight alternatives to mild steel.
Aluminum and AHSS alloys do not, however, have the same degree of ductility and resistance to work hardening offered by mild steel. Forming a flange on a sheet metal body panel made of aluminum or AHSS alloys is more difficult than forming the same flange on a mild steel panel due to the reduced ductility of aluminum or AHSS alloys. One proposed solution to this problem is to form a larger radius hem when making body panels of aluminum sheet metal. Larger radius hems result in lower fit and finish ratings because larger radius hems may cause gaps to appear larger between door closure panels and their openings.
The low ductility of aluminum may cause tears or splits starting from the outer surface of a hem. Tears and splits result in high part rejection rates and unacceptable scrap rates.
Substantial work hardening may occur during the hem flange formation process. The hem flange formation process is the initial step in forming a hem wherein a peripheral portion of a blank or drawn part is bent to about 90 degrees. Forming a 90 degree bend in an aluminum sheet around a relatively tight radius causes substantial amounts of deformation. Stretching the trimmed surface may lead to edge cracking. This amount of strain may result in splits and even tears as the hem flange is further formed in pre-hem and final hem forming steps.
Flanging and hemming of aluminum panels often requires larger radii due to insufficient formability of aluminum alloys (6111-T4; 6022-T4; 6016-T4), advanced high strength steel (AHSS) (DP500 steel) and similar materials for outer skin panels. One of the major problems for implementation of AHSS and aluminum alloys for outer skin panels is splitting of the sheet material from the trimmed surface in stretch flanging and stretch hemming areas. Attempts to reduce the radius of a hem or flange have resulted in splits along the flanging line. Applicants have proposed a two-step flanging operation in which a large radius bend is first made and then a smaller radius bend is made on the larger radius bend. A cam former that requires a complex tooling arrangement may be used to form a smaller radius bend after a larger radius bend is made in a normal flange forming die.
A simpler tooling configuration would be preferable that could obtain sharp flanging in a single step. A tooling solution would be preferred that would facilitate combining and simplifying the steps of drawing, trimming and flanging. There is a need for a flange forming and trimming tool that can form a sharper peripheral radii on a flange in one step without employing an expensive cam mechanism and without requiring an extra stamping operation.
Flange splitting from the sheared surface is a barrier to implementation of higher strength lower guage AHSS steels, such as DP500, for outer skin panels in auto industry. It is also a reason for the limited implementation of aluminum on vehicles. The use of aluminum in vehicle body parts has frequently been limited to hoods, with substantial difficulties being encountered when it is attempted to use aluminum for fenders and decklids.
In conventional sheet metal forming operations a flange is first trimmed and then flanged. Tooling dies may become contaminated with slivers that are formed when a flange splits. Slivers can be spread to the subsequent operations from the trimming operation because it is usually not a final part forming operation. Elimination of slivers is important for outer skin panels that must have a high quality class A surface.
Another problem is that aluminum or AHSS alloy panels tend to spring-back elastically after cold forming. Spring-back can be accommodated and remedied by re-striking the panel to eliminate stresses in the metal that cause spring-back.
These and other problems are addressed by Applicant's disclosure as summarized below.